The invention relates generally to geological test instruments, and, in particular, to instruments for detecting how well fluids are conducted through geological materials such as clays, rocks, and soil.
The hydraulic conductivity of geological materials is important for determining their drainage capacity and the mobility of liquids such as oil or water in subsurface strata. The hydraulic conductivity of less permeable geological media may be less than 10.sup.-4 cm/sec.
Measurement of such hydraulic conductivities presently entails placing the sample in a test cell so that liquid flow may occur only between an inlet and outlet opening in the cell. Tubing connects the openings to burettes containing the test fluid, typically water. The burettes have adjustable heights and graduation lines so that the column heights of the contained fluid may be easily measured.
In the "fallinghead risinghead" method of measuring hydraulic conductivity, the water level in one burette is placed above the water level in the other to establish a pressure differential across the sample and the rate of flow measured by comparing, at periodic intervals, the changing heights of the liquid columns in the burette.
In the "fallinghead" method, only one burette is used and the remaining opening in the test cell drains into a graduated cylinder or similar device.
In both methods, the burettes are typically open to the air. However, they may be closed and pressurized, for example with compressed air, to achieve a greater pressure difference. This pressurization addresses the problem that steady state hydraulic flow necessary to establish conductivity can take many hours or weeks to occur.
In a "constant volume" method originally described by Bjerrum, L. and Huder, J., in their publication Measurement of Permeability of Compact Clays, Proc. 4.sup.th Intl. Conf. on Soil Mech. and Foundation Eng., 1957, a closed loop is established between the inlet and outlet to and from the test sample so that the test sample always has a constant volume of test fluid. A falling column of mercury incorporated into this closed loop provides the pressure difference across the test sample. While this method has decreased measurement times (by decreasing the time to steady state hydraulic conductivity where inflow equals outflow), the test results can still take many hours or days to stabilize.
All of these methods have produced erratic results if insufficient time is allowed for stabilization to occur.